Although female dogs start going into heat at the age of 6 months, they should not be bred until their second season, which is usually six months later. Males reach sexual maturity sometime between the ages of 12 to 15 m... More »

Photos of large dog breeds can be found at Animal Planet, DogBreedsList.info, and DogBreedInfo.com by searching for large dog breeds either in the search bar or selecting the correct tab on the homepage. Each website fea... More »

www.reference.com Pets & Animals Pets Dogs

Some extra-large dog breeds are mastiff, great Dane, Newfoundland and great Pyreness. There are multiple types of mastiff dogs, including a regular mastiff, Neopolitan mastiff and bull mastiff. These are all very large d... More »

www.reference.com Pets & Animals Pets Dogs

Canines typically menstruate twice each year, with most coming into heat every 6 months, although menstrual rates vary among breeds. Small dogs typically begin menstruating at earlier ages than female canines of larger b... More »

The fertility of male dogs varies depending on breed and size, but most male dogs begin to reach sexual maturity at six months of age. It often takes a few extra months for larger dogs to become fertile. More »

www.reference.com Pets & Animals Pets Dogs

Female Labrador Retrievers go into their first heat between 6 and 24 months of age, which is the same age range that all dogs are given. The Labrador Retriever breed tends to be toward the latter end of this age range du... More »

Although a female Shih Tzu has her first heat at 6 months, AllShihTzu.com states that it is not safe to breed her until her second heat at least. She should only have three litters total and stop breeding once she turns ... More »